Selective photocatalytic reduction of CO2 to CO mediated by a [FeFe]-hydrogenase model with a 1,2-phenylene S-to-S bridge

Photocatalytic reduction of CO 2 to CO is a promising approach for storing solar energy in chemicals and mitigating the greenhouse effect of CO 2 . Our recent studies revealed that [( μ -bdt)Fe 2 (CO) 6 ] ( 1 , bdt = benzene-1,2-dithiolato), a [FeFe]-hydrogenase model with a rigid and conjugate S-to-S bridge, was catalytically active for the selective photochemical reduction of CO 2 to CO, while its analogous complex [( μ -edt)Fe 2 (CO) 6 ] (2, edt = ethane-1,2-dithiolato) was inactive. In this study, it was found that the turnover number of 1 for CO evolution reached 710 for the 1 /[Ru(bpy) 3 ] 2+ /BIH (BIH = 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[ d ]-imidazole) system under optimal conditions over 4.5 h of visible-light irradiation, with a turnover frequency of 7.12 min −1 in the first hour, a high selectivity of 97% for CO, and an internal quantum yield of 2.8%. Interestingly, the catalytic selectivity of 1 can be adjusted and even completely switched in a facile manner between the photochemical reductions of CO 2 to CO and of protons to H 2 simply by adding different amounts of triethanolamine to the catalytic system. The electron transfer in the photocatalytic system was studied by steady-state fluorescence and transient absorption spectroscopy, and a plausible mechanism for the photocatalytic reaction was proposed.